Acronym for interface message processor. One of the switching computers that
together form(ed) the backbone network for the ARPANET. The original IMPs were
Honeywell 516 minicomputers with special interfaces for 56 kilobits per second
leased lines. The current IMPs are BBN C/30 minicomputers; other minicomputers
used include the Honeywell 316 and the Lockheed Sue (Pluribus IMP). The primary
function of the IMPs is to forward packets between the ARPANET hosts using 56
Kbps leased lines. Specially configured IMPs have, however, performed other
functions in the ARPANET. The most important of these are the TIPs (terminal
interface (message) processors). TIPs are IMPs with additional hardware and
software to provide network access for asynchronous terminals at speeds up to
1200 baud. The TIP thus functions as a small host computer in addition to its
packet-switching responsibilities as an IMP.
Other special IMP configurations include satellite IMPs with extra memory
buffers for high-delay satellite links, IMPs with message encryption hardware
for secure communication, and IMPs that run special control programs for
high-bandwidth packet speech and video traffic.